Method of making an incandescent lamp



June 15, 1965 N. P. DEMAS 3,189,395

METHOD OF MAKING AN INCANDESGENT LAMP Filed Aug. 29, 1961 .1. .24. 26 T IM- l N V E N TO R Mara/45,905 145 ATTO F! N EY5 United States Patent C The present invention relates to the manufacture of lamps, radio tubes and the like and comprises a novel proces which, as compared to processes heretofore in use, requires fewer steps and results in a lamp or tube having longer and more eflicient life. The invention in cludes also the new products produced by the process of the invention.

A the invention, and the advantages thereof, can best be understood by a comparison of the new process with corresponding stages of conventional lamp making procedure, a brief description of such conventional procedure as applied to miniature incandescent lamps will first be given.

In conventional manufacture of miniature incandescent lamps, filament leads carrying gettering material, for example aluminum and zirconium powder suspended in amyl acetate, are mounted in a glass bead and a tungsten filament carrying phosphorous nitride or other gettering material is secured to the leads. After the lead and filament assembly is sealed through the neck of a glass bulb and before tipping of the exhaust tube, the bulb is treated on a twenty-four stage rotary heading machine to which four pumps are connected. The bulb is indexed from stage to stage, being alternately pumped out and flushed with inert gas. About fourteen stages of the machine are pumping stations. After the various processes on the machine are completed and before tipping off of the exhaust tube, the filament is energized to perform the gettering operation. Before shipment of the lamps they are subjected to a spark coil for detection of any leakage in the seal or contamination of the gas fill in the bulb. If any water vapor is present in the bulb it will react during use with the tungsten of the filament forming tungsten oxide and hydrogen. The tungsten oxide deposits on the wall of the bulb and thereafter reacts with the hydrogen to form additional water vapor, leaving a tungsten deposit on the Wall which blackens the wall. The cycle then repeats with the newly formed water vapor reacting with the hot tungsten of the filament.

The new process when applied to incandescent lamp manufacture makes it possible to eliminate about seven pumping stages from the heading machine and to operate the machine with two rather than four pumps thus saving time and equipment in the manufacture. Furthermore, the process of the invention makes it unnecessary to utilize the spark coil test as only visual inspection i necessary to determine whether or not there is leakage or contamination of the gas fill in the lamp bulb. These major improvements in the process of manufacture of an incandescent lamp are brought about through the substitution of pure alkali metal, such as sodium, as the getter and more particularly by the novel method of introduction of the getter into the lamp bulb. This novel method is predicated upon the known fact that an alkali metal silica glass when heated to a relatively high temperature becomes an electrolyte so that if a source of ions of the same alkali metal as is in the glass is brought into contact with the heated glass the ions will migrate through the glass when subjected to an electric field. Prior to applicants invention it was not appreciated that such migration of alkali metal through heated glass could be made sufficiently rapid to be useful in lamp production or that a pure alkali metal could be such an efficient getter that Patented June l5, 1965 addition of other getter materials to the tungsten filament and leads was not necessary.

In practicing the new method, a molten compound of the alkali metal, for example, sodium nitrate or sodium nitrite when the lamp bulb is soda glass, is brought into contact with a part of the wall of a partially completed lamp that has been heated. A potential difference is applied across the filament and molten bath while a predetermined sub-atmospheric pressure is maintained within the lamp bulb. Electron emission is obtained either by heating the filament or a a result of glow discharge. Sodium ions from the molten bath then migrate through the glass wall and attract the electrons emitted by the filament which thereupon neutralize the sodium ions into sodium atoms. The sodium atoms vaporize at the temperature of the heated glass and condense upon engagement with the unheated part of the bulb. By controlling the potential difference bet-ween the filament and molten bath and the temperature of the bath and wall an accurate control of the quantity of sodium introduced into each lamp may be obtained. After the bulb containing the deposited sodium is tipped off a white ring of sodium i visible in the completed lamp if, and only if, there is no leakage in the seal or harmful impurities in the gas fill. Thus the presence of the white sodium ring is an indication that the bulb is gas tight and will have a long and efficient life. shipment of the lamp.

For a better understanding of the invention and of the preferred method of practicing the invention reference may be had to the accompanying drawing of which:

FIG. 1 is a diagram representing a stage in the process of the invention wherein a part of the Wall of a partially completed lamp is heated to render the glass conductive;

FIG. 2 is a diagram representing the succeeding stage of the process wherein alkali metal ions are caused to migrate through the heated portion of the lamp bulb;

FIG. 3 is a diagram representing an alternative and preferred arrangement for causing migration of the alkali metal ions through the heated portion of the lamp bulb; and

FIG. 4 is a view of a tipped off lamp produced by the process of the invention, the lamp being shown prior to the basing operation.

In FIG. 1 a partially completed lamp 2 is shown as comprising a bulb 4, lead-in wires 6 sealed through the neck 8 and anchored in a glass head 10, and a filament 12. The neck 8 is connected through a restricted section 14 to the exhaust tube 16. The lamp is mounted by conventional means, not shown, at a position on the conventional exhaust machine with its exhaust tube 16 coupled through valves (not shown), to the pumping system. The bulb previous to the position illustrated in FIG. 1 has been alternately evacuated and flushed with clean inert gas such as argon or nitrogen. At the position shown in FIG. 1 the bulb is filled to at least mm. Hg pressure with inert gas. The lower half or more of the bulb is then heated, as by a flame from a gas burner 18, to a temperature sufiiciently high to make the glass conductive but below the softening temperature of the glass. For a lime or soda glass, a temperature of about 300 C. is suitable.

The lamp is then indexed to the next position on the machine which may be that of FIG. 2. In this position, while the lower half of the bulb wall is still hot, the exhaust tube is connected to the pumping system and the pressure in the bulb reduced to about 10 mm. Hg and a reservoir 24) containing molten sodium nitrate is brought into engagement with the heated portion of the bulb. The reservoir is connected through a switch 22 to the positive terminal of a source of energy 24, the negative No spark test is necessary before through cesium glass.

3 terminal of which is connectec to one of the leads 6. A second source of energy 25, is connected across the leads 6 to energize the filament. Since sodium nitrate or sodium nitrite dissociates into Na+ and NO or N@2 ions in the molten state, the positive sodium ions will migrate into and through the hot glass wall of the lamp bulb under the influence of the negative potential on the filament caused by the source 24. The energized filament 12 emits electrons which will move toward the reservoir under the influence of the electrostatic field set up by the source 2-4. As the sodium ions arrive at the inside surface of the bulb, the electrons neutralize the ions, reducing the ions to neutral sodium atoms. At the reduced pressure in the bulb, the temperature of vaporization of sodium is below that of the heated glass wall of the bulb and hence the sodium atoms will vaporize and move away from the hot zone. When they strike the cooler, upper half of the bulb they condense thereon to form a coating indicated diagrammatically at 258. The quantity of sodium so deposited within the bulb may be varied by varying the magnitude of the source 24 and by varying the temperature of the molten bath. For a miniature lamp a sodium deposit of 40 micrograms has been found satisfactory for getter operation.

In the process of FIG. 2 the filament is energized to emit electrons thermionically. Thermionic emission is are not necessary if the gas in the bulb is readily ionizable at the particular pressure within the bulb. For example, if the gas is argon or neon at a pressure of the order of 100 to 200 mm. Hg ionization will start within the bulb when the potential between the hot glass and the filament is of the order of a few hundred volts. Such ionization causes cold emission of electrons from the filament sufficient for neutralization of the sodium ions migrating through the glass wall.

The alternative circuit of FIG. 3 insures a very accurate control of the quantity of alkali metal deposited on the inner walls of the bulb. in this arrangement a capacitor 363, which may be a 1009 microfarad condenser, is first charged by the source 24, in this case a 100 volt supply battery, and then the condenser is disconnected from the battery and connected across the filament and reservoir as by sequential or conjoint opening of switch 22 and closing of a switch 32. A filament supply battery has not been shown in FIG. 3 but could be provided if thermionic emission is desired.

The lamp, after the coating has been formed, either as described in connection with PEG. 2 or as described in connection with BIG. 3, is ready to be gas filled or tipped off under vacuum at the constriction 14. In the completed but unbased lamp illustrated in FIG. 4 the presence of the white sodium deposit 2% indicates in the case of a gas filled lamp that the gas is free of harmful constituents and in the case of a vacuum lamp that the vacuum seal is tight. In either case the manufacturer and purchaser can be confident that a completed lamp in which the white sodium deposit is visible will not blacken and will have a long useful life.

The new process has been described with specific reference to sodium as the getter and with particular reference to the manufacture of miniature incandescent lamps. Other alkali metals can be substituted for sodium provided the glass wall contains the same alkali metal. Potassium ions can be introduced through potash glass, lithium ions through lithium glass and cesium ions From a practical standpoint, sodium is the preferred material for the new process. Sodium nitrate and sodium nitrite are relatively inexpensive and are safe to handle. Likewise soda glass is a particularly suitable glass for lamps and is conveniently employed. Although the drawing illustrates the invention as applied to the manufacture of miniature incandescent lamps, obviously the process is equally applicable to lamps of any size, to glass walled radio tubes and to gas discharge cold cathode devices.

In the process of the invention the sodium, or other alkali metal, when it reaches the inner surface of the hot glass wall of the lamp envelope, is chemically pure, irrespective of the purity of the compound from which it is extracted. This contributes to the practicability of the process as it makes unnecessary preliminary purification of the compound. If an alkali metal for use as a getter were introduced into the envelope in any other way, for example by decomposition within the envelope of a compound introduced through a side tube later sealed off, great care would be required to insure the purity of the compound. Also the products of decomposition, other than the desired getter, would have to be thoroughly pumped out, increasing rather than reducing, the number of required pumping stations of the exhaust machine.

It is believed that the main reason why the process of the invention requires fewer pumping operations than in conventional practice, is because the omission of the usual getters, with the binders therefor, makes it unnecessary to remove the binder materials after the gettering operation.

Pressures within the lamp envelope of 10 to 260 mm Hg during introduction of the alkali met-a1 into the envelope through the hot portion of the wall thereof have been specified by Way of example. Other pressures may be employed provided certain criteria are met. The pressure must not be so high that the alkali metal will not vaporize at the temperature of the envelope where in contact with the molten bath. On the other hand, the pressure should be high enough to provide some ionization of the gas within the envelope as such ionization appears to promote the action and insure rapid deposition of the alkali metal.

From the foregoing description it will be apparent that the invention comprises a novel, economical and commercially practical method of manufacturing lamps, radio tubes and the like and novel products manufactured by such procses that incorporate a visible indication of tightness of seal and purity of contents.

The following is claimed:

1. In the manufacture of an incandescent lamp, a method for introducing a getter into the lamp and for eliminating the usual spark test given the lamp, comprising making the glass envelope for the lamp of an alkali metal glass and with the filament for the lamp therein and the terminals for the filament extending therefrom, locally heating a portion of the glass envelope While it is unsealed to a temperature which is sutficient to cause the glass thereof to become electrically conductive but which is below the softening temperature of the glass, bringing the heated portion of the unsealed glass envelope into contact with a molten compound of the alkali metal while maintaining a subatmospheric pressure of inert gas within the envelope, initiating electron emission from the lamp filament and establishing a potential difference between the lamp filament and the molten compound while the heated portion of the unsealed glass envelope is in contact with the molten compound to cause alkali metal ions to migrate through the glass envelope and deposit as a getter on unheated portions of the envelope after neutralization by electrons given off by the filament, hermetically sealing the glass envelope with the filament for the lamp therein and the terminals for the lamp extending therefrom while maintaining subatmospheric pressure within the glass envelope, and examining the unheated portions of the glass envelope for the deposit of the getter to determine whether the lamp is hermetically sealed.

2. The method according to claim 1 wherein said potential difference is obtained by first charging a condenser to a predetermined voltage and then connecting the charged condenser across the molten compound and the filament whereby control is obtained of the amount of alkali metal deposited.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Graaff 316--3 Compton 313-174 Zworykin 313-317 Trenzen 3163 Wade 313-222 Roth et al. 313222 6 OTHER REFERENCES ARTHUR GAUSS, Examiner. 

1. IN THE MANUFACTURE OF AN INCANDENSCENT LAMP,A METHOD FOR INTRODUCING A GETTER INTO THE LAMP AND FOR ELIMINATING THE USUAL SPARK TEST GIVEN THE LAMP, COMPRISING MAKING THE GLASS ENVELOPE FOR THE LAMP OF AN ALKALI METAL GLASS AND WITH THE FILAMENT FOR THE LAMP THEREIN AND THE TERMINALS FOR THE FILAMENT EXTENDING THEREFROM, LOCALLY HEATING A PORTION OF THE GLASS ENVELOPE WHILE IT IS SEALED TO A TEMPERATURE WHICH IS SUFFICIENT TO CAUSE THE GLASS THEREOF TO BECOME ELECTRICALLY CONDUCTIVE BUT WHICH IS BELOW THE SOFTENING TEMPERATURE OF THE GLASS, BRINGING THE HEATED PORTION OF THE UNSEALED GLASS ENVELOPE INTO CONTACT WITH A MOLTEN COMPOUND OF THE ALKALI METAL WHILE MAINTAINING A SUBATMOSPHERIC PRESSURE OF INERT GAS WITHIN THE ENVELOPE, INITIATING ELECTRON EMISSION FROM THE LAMP FILAMENT AND ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN THE LAMP FILAMENT AND THE MOLTEN COMPOUND WHILE THE HEATED PORTION OF THE UNSEALED GLASS ENVELOPE IS IN CONTACT WITH THE MOLTEN COMPOUND TO CAUSE ALKALI METAL IONS TO MIGRATE THROUGH THE GLASS ENVELOPE AND DEPOSIT AS A GETTER ON UNHEATED PORTIONS OF THE ENVELOPE AFTER NEUTRALIZATION BY ELECTRONS GIVEN OFF BY THE FILAMENT, HERMETICALLY SEALING THE GLASS ENVELOPE WITH THE FILAMENT FOR THE LAMP THEREIN AND THE TERMINALS FOR THE LAMP EXTENDING THEREFROM WHILE MAINTAINING SUBATMOSPHERIC PRESSURE WITHIN THE GLASS ENVELOPE, AND EXAMINING THE UNHEATED PORTIONS OF THE GLASS ENVELOPE FOR THE DEPOSIT OF THE GETTER TO DETERMINE WHETHER THE LAMP IS HERMETICALLY SEALED. 